1. Field of the Invention
The present invention relates to a thermal transfer printer and method for printing pictures and characters, and more particularly to a thermal transfer printer which precisely conveys paper, reduces printing time, size and weight of the apparatus, and is suitable for video color printers and color facsimiles.
2. Background
In a conventional thermal transfer printer, there are four different designs for conveying paper as respectively illustrated in FIGS. 1A, 1B, 1C and 1D.
FIG. 1A illustrates a conventional design using a platen wherein printing is accomplished by conveying a paper sheet 3 at a constant speed due to the frictional engagement of the paper on drum 1 resulting from the application of pressure by thermal print head 2. This design is, however, disadvantageous because, during the conveying, the paper tends to slip as thermal transfer head 2 is unable to apply a sufficient pressure on the paper, thereby greatly distorting the three colors of yellow, magenta and cyan. It is noted that reference numeral 4 represents an ink ribbon and 5a represents guiding rollers.
FIG. 1B illustrates another conventional design using a drum. In order to overcome the problem of the platen design, a holder 6 for holding the end of paper sheet 3 is mounted on a drum 1. However, this design requires a large drum so as to enable to paper sheet 3 to be wound around drum 1 only a single revolution, resulting in a relatively heavy apparatus. In FIG. 1B, reference numeral 2 also represents a thermal transfer head and reference numeral 4 represents an ink ribbon.
In order to solve these problems, there have been proposed a capstan design and gripper design respectively illustrated in FIGS. 1C and 1D. In each of these designs, a drum 1 functions only to support paper sheet 3. In the capstan design the paper is conveyed by a capstan 7 and pinch roller 8 while in the gripper design the paper is conveyed by the reciprocating movement of a gripper 9. Reference numerals 5b, 5c and 5d denote guiding rollers. According to these designs, the paper must be reciprocated back and forth to successively print each color and therefore the printing time to print three colors of yellow, magenta and cyan is relatively long.